For Portugal, I wasn't in particular thinking Salazar would be overthrown. Greece is in a somewhat difficult position given Mussolini's ambitions, how they will be resolved remains to be seen. Holland is coming under increasingly heavy German influence, but Sweden less so, at least at the moment. Switzerland will remain relatively unaffected.
I would think Portugal would be the perfect "Neutral" partner that would be able to import export and be an intermediary between countries in South America. Sweden and Switzerland are basically minor mirrors of the Portugal situation.
Greece should be the farthest thing from Mussolini's mind as he now has the whole NE corner of Africa. His raw material discoveries in Libia and the acquisition of Egypt should put the Italian economy on firm footing. Plus his resources should be spent improving the infrastructure in this area. He also still needs major improvement in his armed forces from and equipment and training standpoint.
This is an interesting possible course of action, and one which if pursued would in time result in considerable gains, but my sense is that the German leadership would be reluctant to prioritize such an effort at the same time as the coming war. As such, it represents an opportunity which Wagner does (at least for the time being) miss.
You're right. This undertaking would be difficult while you're in a shooting war. This would have been needed to be planned out prior to the invasion of Poland and then phased in over the proceeding years. Where this will be very profitable is in the Polish territory with large relatively flat farmland and then in the Ukraine area. They could settle that area kind of how America did with the west; every German moving out there gets 200 acres and a tractor! That would kick start your food production quickly.